Arc discharge tube having an electrode which contains a light-emitting metal



Oct. 8, 1968 KQURY ET AL 3,405,303

. ARC DISCHARGE TUBE HAVING AN ELECTRODE WHICH CONTAINS A LIGHTEMITTINGMETAL Filed D60. 29, 1964 FREDERIC KOURY WILFRID G. MATHESON JOHN FWAYMOUTH INVENTORS ATT NEY United States Patent 3,405,303 ARC DISCHARGETUBE HAVING AN ELECTRODE WHICH CONTAINS A LIGHT-EMITTING METAL FredericKoury, Lexington, and Wilfrid G. Matheson and John F. Waymouth,Marblehead, Mass., assignors to Sylvania Electric Products Inc., acorporation of Delaware Filed Dec. 29, 1964, Ser. No. 421,841

3 Claims. (Cl. 313217) This invention relates to high pressure electricdischarge devices and their cathodes and particularly to devices whichemit colors other than the characteristic bluish hue of mercury lamps.

High pressure electric discharge devices have been known to the art.Such devices have commonly been made with a fill of mercury metal. Whenthe devices are operated, the mercury is ionized and characteristicemission lines of mercury are emitted at 4048, 4348, 5461, 5770 and 5990A. Basically these lines produces a bluish emission. Recently it hasbeen discovered that if iodine and a light-producing metal areincorporated into the arc tube, a wide variety of new emission colorscan be obtained. For example when thorium, scan-dium, vanadium and/orcertain of the rare earth metals such as yttrium, lanthanum, lutetium,holmium, thulium, cerium,'neodymium, praseodymium, gadolinium, terbium,dysprosium and erbium are added to the arc tube, white light can beobtained. If thallium, for example, is added to the arc tube, abrilliant green color is produce-d and when indium, molybdenum andgallium are included blue light is realized. Other metals which havebeen added to the arc tube to produce variations in colors includecadmium and sodium. For example, cadmium produces red and sodium addsyellow.

As with the advent of any new device, however, some engineering problemshave developed. Foremost among these problems has been a continual risein the operating voltage of the lamps during their life. Although thelamps may operate on reasonable voltage when orginally made, duringtheir life the voltage tends to increase. We have now discovered that,to a degree, this increase in operating voltage can be traced to aso-called gettering of one of the several materials in the arc tube.When the metal is gettered or is made chemically ineffective in the arctube, the operating voltage will change. A consumption of the metalcauses an imbalance of the materials in the arc tube, some of whichcause the operating voltage to rise and others of which cause it todecrease. For example, sodium is gettered more quickly than thorium inan arc tube containing sodium, thorium, iodine and inercury. While theratio of sodium to thorium is shifting, the operating voltage will rise.In order to circumvent this problem, we have discovered that if thelight-emitting material is dispensed or metered-out during the operationof the lamp, that the operating voltage will not increase soprecipitously. By our invention, we dispense the light emitting materialby using a tungsten cathode having a conventional external shape butwhich is 'hollow. A requisite quantity of light emitting metal isdisposed in the hollow as a core. During the life of the lamp, the heatof the arc discharge and the chemical reaction of ionic iodidesgradually causes the light-emitting metal to enter into the arc stream.When sufficient quantities of lightemitting material are included as thecore, the gradual getterin-g of this metal will not produce a shift inthe ratios.

Accordingly, the primary object of our invention is the reduction ofincreases in operating voltage of high pressure electric dischargedevices containing mercury iodine and light-emitting metal.

A feature of our invention is the inclusion of a light- 3,405,303Patented Oct. 8, 1968 "ice emitting metal as a core in at least oneelectrode of a high pressure electric discharge device emitting-lightother than characteristic blue hue of mercury lamps.

The other objects, features and advantages of our invention will becomemanifest to those conversant with the art upon reading the followingspecification when taken in conjunction with the accompanying drawingswherein specific embodiments of our invention are shown and described byway of illustrative examples.

Of these drawings:

FIGURE 1 is a plan view of an arc tube of a high pressure electricdischarge device having electrodes fabricated according to ourinvention. 7

FIGURE 2 is an expanded, cross-sectional view of one end of an arc tubeand the electrode.

Referring now to FIGURE 1 of the drawings, the arc tube in general ismade of quartz, although other types of glass may be used such asalumina glass or Vycor, the latter being a glass of substantially puresilica. Sealed in the arc tube at opposite ends thereof are maindischarge electrodes 1 and 2. These electrodes are supported uponlead-in wires 3 and 4 which extend into the electrodes as will bedescribed later. Each of the lead-in wires 3 and 4 are spot welded oraffixed to molybdenum foil sections 5 and 6 which are press sealed inthe glass. A starting probe 7 extends into the arc tube adjacent theelectrode 1 and is used when the lamp is started. Each electrodecomprises rod sections 8 and 9 which usually, although not necessarily,are surrounded by tungsten or molybdenum helixes 10 and 11. A smallquantity of mercury, the light emitting metal, when desired, and asource of iodine 12 is added to the arc tube through an exhaust tube 14(only the fused residual tip of which is shown).

As shown in FIGURE 2, the rod sections extend within a well 15 formed inthe press seal of the arc tube. It is disposed upon lead-in wire 3 whichextends into the rod sections interior. The rod section is formed of atube 16 which is filled with a light-emitting metal 17. Thelightemitting metal can extend downwardly through the whole length ofthe tube 16, as desired. Wrapped around the outside of the rod sectionis the wire helix 11 as described previously. Metals utilized as lightemitting sources can include those outlined previously.

In order to fabricate the rod sections we prefer to vapor deposit ashell of tungsten about the core of aforementioned light emitting metal.Frequently a large number of coatings should be applied to make the tubeof tungsten sufiiciently thick to withstand the temperature to which thearc is operated. Other methods of forming the cathode can includedrilling a hole into a core of tungsten and filling the hole with thelight-emitting material.

When the electrodes were fabricated into lamps, the voltage readingwould not increase appreciably over life. initially, the voltage readingwas 109.2 M, at hours it was 118.5 v. and at 468 hours it was 126 v. Theoperating voltage had substantially plateaued and further increaseswould not be marked. In lamps utilizing only conventional electrodes inwhich no core of light-emitting material was utilized, the operatingvoltage was substantially higher than that which was evidenced with theelectrodes of our invention; initially, the voltage was 128 v. and at480 hours it 'had risen to 158 v. Hence not only was the initialoperating voltage higher than that produced with the electrodes of ourinvention, but moreover such increases were maintained during life.

In order to manufacture this lamp, 50 mg. of Hg, 7.5 mg. Hgl 1.0 mg.TlI, 19.5 mg. NaI were placed in the arc tube. An electrode having athorium core was sealed in. In manufacturing, the arc tube filled withthese ingredients baked for7 minutes, flushed with an inert gas andflamed for 5 minutes. Afterwards it was pressurized to 23 mm. of mercuryand the exhaust tube tipped seal the envelope.

It is apparent that modifications and changes may be rrnade within thespirit and scope of the instant invention. It isour intention, however,to be limited only by the scope of the appended claims.

As our invention we claim:

1. An arc tube for a high pressure electric discharge device eomprisingia glass envelope sealed at either end; an electrode sealed at each endthereof; at least one of said electodes compising a tube of tungsten anda core of at least one light-emitting metal selected from the groupconsisting of thorium, scandium, vanadium, yttrium, praseodymium,gadolinium, terbium, dysprosium, enbium, indium, molybdenum, gallium,cadmium and sodium; said envelope containing a fill including halogenatoms and mercury.

01f to i 2. The are tube according to claim 1 'wherein the core oflight-emitting metal is thorium.

3. The are tube according to claim 1 wherein the electrode has a wirehelix wrapped about the tungsten tube.

References Cited UNITED STATES PATENTS 2,030,807 2/1936 Wiegand 3l3-2252,042,195 5/1936 Scott 313'225 2,499,192 2/1950 Lafierty 313'3462,629,836 2/1953 Deri 313-209 2,975,320 3/1961. Knauer 313'346 3,250,9405/1966 Koury 313225 JAMES W. LAWRENCE, Primary Examiner.

R. JUDD, Assistant Examiner.

1. AN ARC TUBE FOR A HIGH PRESSURE ELECTRIC DISCHARGE DEVICE COMPRISING:A GLASS ENVELOPE SEALED AT EITHER END; AN ELECTRODE SEALED AT EACH ENDTHEREOF; AT LEAST ONE OF SAID ELECTRODES COMPRISING A TUBE OF TUNGSTENAND A CORE OF AT LEAST ONE LIGHT-EMITTING METAL SELECTED FROM THE GROUPCONSISTING OF THORIUM, SCANDIUM, VANADIUM, YTTRIUM, PRASEODYMIMUM,GADOLINIUM, TERBIUM, DYSPROSIUM, ERBIUM, INDIUM, MOLYBEDNUM, GALLIUM,CADMIUM AND SODIUM; SAID ENVELOPE CONTAINING A FILL INCLUDING HALOGENATOMS AND MERCURY.